I would need for content to be shown just on Submenu level 2 and their parent (Submenu1). And since the categories are dynamic (more can be added over time) - I can't use targeting specific category ID.

Can you just add the content in a custom field for each post that needs it? Then you can just edit the template file to check if there's a value for a specific CF key and pop it in where needed. If there's no value then nothing shows up.

If the content is to be the same on each page then you can just create the content in static page, and on each of the pages that need that particular content to show up on just create a custom field called 'content' with a value of 'true'. In you template file you can check if the meta key 'content' has a value of 'true' and if it does then you run a query to fetch the page_ID that contains the content.